Modernized time-warp a bit with bundler

/cc @bjhess @lorenzoplanas @tjschuck 

* Added bundler so we can build and release new gem versions with it. 
* Also using Minitest 5... because.
* Included a mix of #9 and #7 so that it works with ruby 1.9's `Time.new`.

Once merged we can release a new version.

Cheers!
Dunno if this has already happened, but this implicitly removes support for 1.8.  I know it's EOL'd, but I'd rather be generous for public libs.

If this was working for 1.8 before this PR, it should _probably_ keep working afterwards unless we have a compelling reason to drop support.  (I don't overly consider "new hash syntax" to be compelling.)